More than one-half of the electricity consumed in the U.S. is currently generated by inefficient coal-burning utility plants. In spite of the increased use of oil and gas resources, the low cost, ready and widespread availability, large reserves of coal, and large coal industry workforce ensure that coal will remain a significant resource for industrial purposes, and, especially, for worldwide energy generation for the foreseeable future. However, coals are extremely diverse and heterogeneous and generally contain significant amounts of moisture, ash, sulfur, and mineral impurities, all of which detract from its current potential to be an efficient and clean-burning fuel.
The literature of science and technology, process engineering, and patents is replete with failed attempts to improve the overall combustion properties of solid fuels. Physical and chemical methods and even biological organisms are used in coal cleaning and waste removal. Coal washing is widely used to remove the higher specific gravity ash and pyritic sulfur, but typically results in the loss of up to 20% of the coal in the process. Various but costly methods of coal drying are used in an effort to increase the BTU/lb (British Thermal Units per pound) of the coal, and many methods have been investigated in an effort to significantly reduce sulfur.
The burning or combustion of coal, even higher ranks such as anthracite and bituminous, generates significant emissions and is of increasing concern due to environmental and global warming considerations. The burning of high-sulfur coals has inflicted a heavy toll on the atmosphere in eastern Europe, China, and elsewhere, and is in large measure responsible for the widespread but unsuccessful efforts over the past 25 years to find an efficient and affordable means of desulfurization. Much of this effort has involved electromagnetics; viz, the use of ultraviolet, optical, infrared, radio, microwave, x-ray, and even gamma ray frequencies and combinations thereof. Most have met with failure. In fact, there is to our knowledge no single, effective and economically viable pre-burning process technology in commercial practice, electromagnetic or other, for removing sulfur or any of the other major constituents of coal.
The presence of moisture, ash, sulfur and other constituents in varied amounts in all coals results in a variety of problems when coal is burned or otherwise heated for any purpose. Noxious gases, such as nitrous oxides (NOx) and sulfur oxides (SOx), resulting from coal burning have adverse effects on the environment, including acid rain, smog, unhealthy air laden with sulfur (resulting in the common yellow sky color in large regions of China), and the deposition of toxic particulates, some carried far from their origins by upper air currents. Further, coal burning leaves behind inorganic ash with its trace elements such as mercury, the consequences of which are seldom considered when ash is handled or used as a filler in widespread and long term applications such as road building and repair. There is now added concern due to the large amounts of carbon dioxide (CO2) generated during coal burning, which contributes significantly and directly to global warming. Finally, the presence of significant moisture in many coals results in inefficient burning, leading to the burning of more coal and the consequences of increased emissions thereby. Relatively little emphasis has been placed on de-moisturization (i.e., contributing to the increase in BTU/lb, which directly results in being able to burn smaller amounts of coal to generate the same amount of energy and thereby, in itself, reducing all emissions therefrom).
Numerous attempts have been made over the years to remove or otherwise reduce the amounts of ash and sulfur in coal and thereby reduce the various forms of contamination by improving the quality and combustion characteristics of coals. Unfortunately, such attempts have proved to be time consuming, costly, and impractical.
For example, attempts have been made to “wash” the coal prior to combustion. These attempts can be expensive and require extensive plant equipment additions and modifications. In washing systems, the coal must be crushed or sized using screens, pulverizers, ball mills, crushers, or other similar types of grinding equipment prior to being fed to the washer. Typically, reducing the size of the coal using such types of equipment involves relatively heavy and large equipment that is expensive to purchase, maintain, and operate.
In other examples, cleaning systems and methods involve extensively drying the coal using centrifuges, rotary drum filters, fluidized bed dryers or other similar types of drying equipment prior to burning or combusting the coal or fuel. Typically, drying the coal using such types of equipment involves complex or multiple stages of equipment that are also expensive to purchase, to maintain, and to operate.
Other attempts utilize the addition of one or more catalysts to the coal, in an effort to reduce the amount of unwanted by-products created in the combustion of the coal. For example, certain types of catalysts added to coal can reduce the amount of sulfur emitted during the combustion of the coal. These attempts are also aimed at improving the combustion characteristics, such as increasing the BTU/lb, of the coal to be burned. However, these attempts can create a different set of by-products that can be hazardous and expensive to dispose of or to store.
In the absence of pre-burning solutions to this long-standing problem, post-burning scrubbing is currently the preferred methodology of the coal-fired electric utility industry. For example, scrubbers have been installed to clean post-combustion flue gases, removing SOx and NO, compounds from the flue gases. This type of equipment and other similar systems and processes are expensive to install, maintain and operate. Unfortunately, this does not solve the CO2 problem, and recent efforts to capture and re-route CO2 gases do not appear promising or practical.
Finally, many coal-burning utilities blend low-sulfur coal with high-sulfur coal to reduce overall sulfur so as to meet the government-regulated index of pounds of SO2 per million BTUs. This generally requires long distance transportation of heavy, moisture-laden coals with transportation costs often equaling or exceeding the cost of the coal.